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Having picked up some cheap boost wheels I’m thinking of building my spares into a hardtail to take over kid ferrying duty from a loathsome specialized Crosstrail. The only part that is missing right now is a specific boost chainset. Can I just fudge the extra 3mm chain line with some washers or will I run into clearance issues on the crank spider?
Depends on frame and chainset but, I'd try what you have and see if it works.
You'll know pretty quickly if it is going to foul the chainstay.
I had 3 boost frames and used normal cranksets with a 30 or 32t ring in the middle position. Only the latest frame needed the proper boost chainring option due to fouling.
If you have older Shimano cranks, you could move the ring from the inside of the tabs to the outside to give a bit more clearance.